Urban emissions and expansion intensify extreme precipitation and flood susceptibility in Texas.

The Texas Triangle is rapidly expanding and increasingly vulnerable to extreme rainfall and floods. Using the Weather Research and Forecasting model coupled with chemistry (WRF-Chem), we simulate a cold-season frontal storm and a prolonged warm-season event, comparing a present-day configuration with an end-of-century urban-expansion scenario consistent with Shared Socioeconomic Pathway 5 projections. Across scenarios, meteorological boundary conditions are held fixed, while urban land cover is expanded and anthropogenic emissions are enhanced accordingly. In the cold-season case, urban growth enhances activation of fine-mode cloud condensation nuclei, strengthens updrafts, and produces statistically significant precipitation increases over San Antonio and Austin. In the warm-season case, the response is heterogeneous, with early enhancement on the western side of Dallas-Fort Worth followed by suppression eastward as the system evolves. Flood-risk analysis, based on the model-diagnosed runoff coefficient and a high-resolution stochastic framework (SCS-CN), reveals a systematic expansion of high-risk zones under future urbanization, amplifying existing runoff hotspots and generating new ones across metropolitan areas. Overall, the consistency between the WRF-Chem runoff diagnostics and the stochastic SCS-CN framework supports the use of WRF-Chem as a first-order, spatially explicit indicator of relative urban flood risk under extreme precipitation.